1. Field of the Invention
The present invention relates to a thermal activation device for heating and thermally activating a heat-sensitive adhesive sheet so as to develop adhesive properties, a printer including the thermal activation device, a thermal activation method, and to a method of producing a self-adhesive label.
2. Description of the Related Art
Up to now, a heat-sensitive adhesive sheet having a heat-sensitive adhesive layer with adhesive properties to be developed by heat has been put to practical use. The heat-sensitive adhesive sheet has some advantages in that, for example, the heat-sensitive adhesive sheet can be easily treated because the sheet has no adhesive properties before being heated, and industrial waste is not produced because releasing paper is not used. In order to develop an adhesive force of the heat-sensitive adhesive layer of the heat-sensitive adhesive sheet, the heat-sensitive adhesive layer is heated using a thermal head, which is generally used as a recording head of a thermal printer, in some cases. In this case, a platen roller is provided so as to be opposed to the thermal head, and the platen roller is rotated while pressing the heat-sensitive adhesive sheet against the thermal head, thereby transporting the heat-sensitive adhesive sheet. Then, an entire surface or a part of the heat-sensitive adhesive sheet is thermally activated, thereby developing the adhesive force.
In addition, JP 2004-243606 A discloses a printer which includes: a recording device (printing unit) for recording desired characters, numeric characters, graphics, and the like on a surface of a continuous-form heat-sensitive adhesive sheet, which is an opposite side of a heat-sensitive adhesive layer thereof; a cutting device (cutting unit) for cutting the heat-sensitive adhesive sheet; and a thermal activation device (thermal activation unit) including the thermal head and the platen roller. In the printer, the heat-sensitive adhesive sheet, which is subjected to recording on one surface thereof by the recording device, is cut into a predetermined length by the cutting device, and the cut heat-sensitive adhesive sheet with a short length is supplied to the thermal activation device. In the thermal activation device, the cut heat-sensitive adhesive with the short length is transported through rotation of the platen roller, and at the same time, is thermally activated by the thermal head. In other words, a portion of the heat-sensitive adhesive sheet, which has passed through the thermal head, is thermally activated to thereby develop adhesive properties thereof. A self-adhesive label, which is produced by the printer and is formed of the heat-sensitive adhesive sheet, is held in a state where a leading edge thereof protrudes from a discharge port. Then, a user picks up the leading edge thereof protruding from the discharge port, and pulls out the self-adhesive label from the discharge port for use.
As described above, in the thermal activation device, the cut heat-sensitive adhesive sheet is transported through the rotation of the platen roller. To be exact, the heat-sensitive adhesive sheet is transported during a time when a trailing edge thereof is in contact with the platen roller, but when the trailing edge of the heat-sensitive adhesive sheet is apart from the platen roller and reaches a position at which the trailing edge thereof is not in contact with the platen roller, a transporting force is not transmitted to the heat-sensitive adhesive sheet, thereby stopping the transportation of the heat-sensitive adhesion sheet. Then, the heat-sensitive adhesive sheet is held in a stationary state until the user pulls out the heat-sensitive adhesive sheet. In this case, when a part (trailing edge) of the heat-sensitive adhesive sheet is in contact with the thermal head, the heat-sensitive adhesive sheet with adhesive properties being already developed is adhered to the thermal head, and is maintained at high temperature. Even when driving of the thermal head is stopped, a residual heat remains in the thermal head, so the heat-sensitive adhesive sheet is continuously excessively applied with heat without being naturally cooled. As a result, the heat-sensitive adhesive layer of the heat-sensitive adhesive sheet is excessively heated, and the adhesive force of a part of the heat-sensitive adhesive layer is reduced. In addition, in a case where a recordable layer is formed on a surface opposite to the heat-sensitive adhesive layer, heat is excessively transmitted to the recordable layer, which may cause abnormal color development, for example, generation of a black line (stripe line).
In order to avoid the above-mentioned problems, the heat-sensitive adhesive sheet may be held so as not to be in contact with the thermal head. However, the platen roller also has a function of pressing the heat-sensitive adhesive sheet against the thermal head when the thermal activation is performed. Accordingly, a heat-generating portion of the thermal head and a portion thereof for transmitting the transporting force to the heat-sensitive adhesive sheet are generally overlapped each other at substantially the same point. In view of the above, it is difficult to set the trailing edge of the heat-sensitive adhesive sheet to be apart from the heat-generating portion of the thermal head as soon as the trailing edge thereof is apart from the platen roller. If the heat-sensitive adhesive sheet can be transported so as to reach a position where the heat-sensitive adhesive sheet is not in contact with the thermal head, the heat-sensitive adhesive sheet is inevitably positioned within a short distance from the thermal head at which the heat of the head of the thermal head is transmitted to the heat-sensitive adhesive sheet. Accordingly, it is impossible to prevent the heat-sensitive adhesive sheet from being partially excessively heated as described above.
Thus, it is difficult to transport the heat-sensitive adhesive sheet so as to reach the position at which the heat of the thermal head is not transmitted to the heat-sensitive adhesive sheet, only by transporting the heat-sensitive adhesive sheet through rotation of the platen roller. In view of the above, another transporting means may be provided at the downstream side of the platen roller (at a position far from the thermal head). For example, between a contact between the thermal head and the platen roller, and the discharge port, a pair of discharge rollers may be provided so as to sandwich the heat-sensitive adhesive sheet therebetween, thereby transporting the heat-sensitive adhesive sheet to a position apart from the thermal head, at which the heat of the thermal head is not transmitted to the heat-sensitive adhesive sheet. However, in this case, there is a risk that the heat-sensitive adhesive sheet which has developed the adhesive force is adhered to the transport roller and cannot be transported to the discharge port. If a force of the pair of discharge rollers for sandwiching the heat-sensitive adhesive sheet is reduced so that the heat-sensitive adhesive sheet is not adhered to the discharge roller, there is a risk that a sufficient transporting force cannot be transmitted to the heat-sensitive adhesive sheet and the heat-sensitive adhesive sheet cannot be transported to the discharge port after all.
A transporting device other than the pair of discharge rollers, for example, a transporting device including a belt conveyor and a movable suction cup, may be provided between a contact between the thermal head and the platen roller, and the discharge port. However, in this case, it is necessary to provide a complicated and large structure between a contact between the thermal head and the platen roller, and the discharge port, with the result that the structure of the device is complicated and the size thereof is increased, and in addition, costs thereof increase. Further, in order for the user to pull out and use the heat-sensitive adhesive sheet which is cut with a short length and made into labels, it is necessary to hold the heat-sensitive adhesive sheet in a state where the trailing edge thereof protrudes to the outside from the discharge port. Accordingly, the heat-sensitive adhesive sheet with the short length is provided only at a position extremely close to the discharge port. For this reason, it is necessary to additionally provide a structure for holding the heat-sensitive adhesive sheet with the short length, in the vicinity of the discharge port. Further, in a case where the distance between the contact between the thermal head and the platen roller, and the discharge port, becomes longer when the transporting device including the belt conveyor and the suction cup is provided, a distance by which the heat-sensitive adhesive sheet is transported also becomes longer.